To evaluate the utility of CA 15.3 in detecting breast cancer relapse before clinical evidence of metastatic disease, serial determinations of CA 15.3 serum levels were made in 444 disease-free patients after surgery, during 36 months of follow-up. The results were compared with carcinoembryonic antigen (CEA) and tissue polypeptide antigen (TPA). The study found CA 15.3 to be more sensitive as an early indicator of relapse than CEA or TPA; moreover, simultaneous determination of the other markers offers no information additional to that obtained by CA 15.3 alone.